(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数...(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数
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(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数...(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数
(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数...
(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数列{bn}的前n项
(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数...(1/2)高二数学题:己知等差数列满足:a3=7,a5十a7=26,{an}前几项和为Sn、令bn=1/(an^2一1),求数
设公差为d
则a5=a3+2d a7=a3+4d
则a7+a5=2a3+6d=26
d=2 a1=3
an=2n+1
bn=1/(an^2-1)=1/(4n^2+4n)
a3=7
a5+a7=2a6=26
a6=13
a6-a3=6=3d, d=2
a1+2d=7=a1+4 a1=3
an=3+2(n-1)=2n+1
bn=1/[an^2-1]=(1/4)(1/n-1/(n+1))
b1=(1/4)(1-1/2)=1/8
b1= (1/4)(1-1/2)
b2=(1/4)(1/2-1/3)
..
bn=(1/4)(1/n-1/(n+1))
设等差数列an的公差为d,a5+a7=(a3+2d)+(a3+4d)=2a3+6d=14+6d=26,所以d=2,
所以an=a3+(n-3)d=7+2(n-3)=2n+1,
bn=1/(an^2-1)=1/(4n^2+4n)